NEW YORK (Reuters Health) - Fish oil capsules are a cheap and easy way to get omega-3 fatty acids, but they don't help pregnant women steer clear of postpartum depression.

Nor do they boost mental development in their babies, according to researchers from Australia who tested the effect of daily supplements during the second half of pregnancy -- a period that spans the growth spurt in the fetus' brain.

The researchers gave more than 2,000 women either vegetable oil or fish oil containing docosahexaenoic acid (DHA), which earlier studies have hinted -- but not proved -- might improve pregnancy outcomes.

"Before DHA supplementation in pregnancy becomes widespread, it is important to know not only if there are benefits, but also of any risks for either the mother or child," Maria Makrides, of Women's and Children's Hospital in North Adelaide, Australia, and colleagues write in the Journal of the American Medical Association.

But they might have been too late. As it turned out, a third of the women they approached for their study were already taking fish oil supplements, in agreement with expert advice.

The good news: The only side effect was belching, which was almost twice as common in women eating fish oil as in those on vegetable oil.

The bad news: Fish oil didn't do much to help women's mood after they gave birth, with both groups hovering around 10 percent in self-reported high levels of depressive symptoms.

What's more, when psychologists evaluated toddlers at 18 months they found no differences in babbling, verbal comprehension and other language development, nor in cognitive measures such as exploring objects and forming concepts.

"There are certainly no strong and convincing benefits of fish oil supplements," said Dr. Emily Oken, who studies nutrition during pregnancy at Harvard Medical School in Boston and wrote an editorial on the new results.

But she added that the findings don't rule out that some women, such as those at high risk of depression, might benefit from fish oil supplements.

"Current recommendations suggest that pregnant women should aim to consume 200 milligrams of DHA per day," said Oken, who was not involved in the new study and has no ties to companies making supplements.

Fish might be a better DHA source than capsules, because they contain other nutrients that might be beneficial as well, said Oken. Perhaps, she added, that is why several observational studies have found better pregnancy outcomes in women who eat lots of seafood.

Fish rich in DHA include salmon, trout, sardines and herring, and Oken said one serving per week is plenty. But she advised that pregnant women avoid large fish that contain lots of mercury, such as swordfish, shark, tilefish and king mackerel.

"If women can't or won't eat fish, then they can consider taking fish oil supplements," Oken told Reuters Health. Such products can be purchased for less than $10.

Duffy MacKay of the Council for Responsible Nutrition, a trade association representing manufacturers and ingredient suppliers of dietary supplements, said DHA is an essential part of our diet, because the body can't make it itself.

The compound, a so-called omega-3 fatty acid, is a building block in cell membranes and is particularly important for brain development.

"I would be really sad to see people abandon getting enough DHA or EPA because of a study," said MacKay, referring to eicosapentaenoic acid, another omega-3.

When pressed, however, he wouldn't say whether taking the supplements could decrease depression risk or boost toddler development. "I do not expect that fish oil works like a drug," MacKay told Reuters Health.

The pills used in the new study contained 800 milligrams of DHA and 100 milligrams of EPA, and were provided by Efamol, a company making dietary supplements.

While the results were largely negative, the researchers did find that women taking fish oil cut their risk of giving birth too early -- before 34 weeks of pregnancy -- from more than two percent to just about one percent. A normal pregnancy lasts about 40 weeks.

On the other hand, they also increased their risk going over their due date.

"I think there are two things that are quite clear," said Oken. "One is that it's safe to take fish oil supplements. The second is that they reduce the rate of preterm births."